Generally, an echo canceler in the hands-free talking equipment is constructed using an adaptive filter and an echo suppressor. The echo suppressor, which is used for suppressing residual echo leftover after elimination with the adaptive filter, is realized by a center clipper or a variable attenuator.
Among them, as for the center clipper, since the amplitude level of its removable residual echo is restricted to certain limits, it cannot be applied to the case where the amplitude level of the residual echo is large. On the other hand, as for the variable attenuator, although it is applicable to the case where the amplitude level of the residual echo is large, it is necessary for using the variable attenuator to provide double talk decision processing for preventing a partial loss of transmission voice, and to control the amount of loss of the variable attenuator in such a manner that the transmission signal is attenuated only in a single talk state of a receiving side.
As an example of such a double talk decision method, there is a method described in Patent Document 1, for example. The decision method described in the document obtains the average value of the level differences between the received signal and the transmission signal in a state in which the received signal is in a speech burst condition; makes the average value an estimated echo gain, and obtains the estimated echo level from the estimated echo gain and the received signal level; and makes a decision that the receiving side is in the single talk if the transmission signal or the residual signal level of the adaptive filter is equivalent to or less than the estimated echo level, and in the double talk if it is greater than that.    Patent Document 1: Japanese Patent No. 3386327.
However, in conditions in which the echo level exceeds the transmission voice level averagely, the level of the transmission signal or of the residual signal becomes equal to or less than the estimated echo level regardless of whether the single talk or double talk at the receiving side, and the decision result always becomes the single talk at the receiving side. This offers a problem of hindering the accurate detection of the double talk.
Such conditions can occur in the case where in positional relationships between a talker, a microphone and a speaker, the distance between the talker and the microphone is not short enough compared with the distance between the microphone and the speaker, or the sound pressure level of received voice reproduced from the speaker is much higher than the sound pressure level of the transmission voice.
The present invention is implemented to solve the foregoing problem. Therefore it is an object of the present invention to provide an echo canceler capable of detecting double talk sections precisely and carrying out echo suppression processing even in conditions in which the echo level exceeds the transmission voice level averagely.